Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body. But rather to skid in sideways, chocolate in one hand, a good Reisling in the other, body thoroughly used up, totally worn out and screaming: WOO HOO, what a ride

I'm going to jump in here and suggest that there may be a different reason for your failure, not at all something diodes of any kind will fix. Though I respect Tim and Jerry on all of this stuff, I'm sitting here wondering where they think there's enough inductance in the mains to generate the kind of inductive kick that will trash contactor contacts. Yes there's plenty of inductance in a contactor's *coil* to cause some havoc in other things (not the contacts though), but in a piece of 0000 cable there's not enough inductance even in a big long run to create the kind of failure you're seeing, especially considering the frequency of closures your solenoid is seeing... ie if it were being turned on and off 100 times an hour on a 24/7 basis, yes inductive spikes may eventually roast the contacts but once or twice a day and not many of those days in a year, I just can't see this being the cause.

My take is that there's too much current being carried by the contacts during closure or opening, probably due to the two battery banks being very different in charge from each other when the contacts are closed or opened, and that 0000 cable. That was suggested early in the thread before it went off to diode-land. A very easy way to fix this would be to replace the 0000 sized wire going to and from the solenoid with some wire of a much smaller gauge, sized to carry solely the current that the alternator can produce. I've been using #6 on my setup for 35,000 miles now and only a simple 100 amp solenoid with total success and zero failure. The theory is that the wire adds enough resistance in the circuit that the current thru the solenoid is somewhat limited to what the alternator can produce, and everything's happy. I have a 100 amp alternator and four T-125's and no matter how dead I make them I NEVER see more than 40 amps going thru that contactor. I have a feeling that the way your system is wired and with the 0000 wire, you may be seeing hundreds of amps going thru yours.

In your case you'd need to make sure that your Trace or some other device isn't trying to get it's major share of power thru the solenoid, etc. The solenoid's purpose should be solely to share the alternator with the two banks of batteries. So the best way to wire it would be take a dedicated piece of #6 wire from bank 1's positive directly to the solenoid, and then another dedicated piece of #6 from the other contact of the solenoid directly to bank 2's positive. This way, the batteries will balance but the resistance of the wire itself will become a current limiting factor that will keep things from going spark-spark inside your solenoid, and the size of the wire itself will preclude any massive load drawing devices from trying to nab power via the contactor....

Bottom line in my humble opinion, yes use diodes on the coils and your stereos will probably be much happier (I've had my coil's inductance take out my stereo prior to my putting diodes there!!) but for this problem diodes are far from the correct solution. An hour and a few pieces of #6 should cure you instantly.CheersGary

Gary, The actual inductance, and energy stored therein may well surprise you. I have about 30 feet of 00 wire between my house battery and the coach battery. This calculates, using a calculator on the www, as 13.75micro henries which at 200 amps is storing over 1/4 of a joule, enough to do serious damage quite easily.RegardsJerry 4107 1120

...My take is that there's too much current being carried by the contacts during closure or opening......A very easy way to fix this would be to replace the 0000 sized wire going to and from the solenoid with some wire of a much smaller gauge, sized to carry solely the current that the alternator can produce......The theory is that the wire adds enough resistance in the circuit that the current thru the solenoid is somewhat limited to what the alternator can produce...

What it sounds like you are suggesting is diliberately using an undersized wire, to "current limit" the link between the two banks. What I don't see here is any mention of a fuse... This is the concern I have.

A wire, no matter how undersized, will attempt to transmit the full power demand of the circuit regardless of its size. If a wire is replicating a "current limiting" effect, this means that the resistance of the wire is preventing further increases in power transfer. How this happens is that at a specific point in the increasing slope of transfer, the wire becomes saturated and the electrons instead of flowing through the wire like a stream - are forced through like a 1500psi pressure-washer hose. This causes the conductor to become hot and its resistance to increase, which eventually (assuming that the load is not continuously too high) will balance and "limit" the ammount of current through the conductor.

Unfortunately this heating of the conductor can melt an insulating jacket well before the current and temerature reach equilibrium, or cause the conductor itself to open (think electrical fire here please). This is the whole point of a fuse - it has a smaller gauge conductor which is designed to open when a sustained current transfer throught its package exceeds the rating of the part. By having a small device in a known location, one can control the failure of the long cable run (ensuring that the fuse opens before the cable gains heat which is too much to dissapate safely). Without a fuse, the heat will build up in a cable where the conductor cannot radiate the heat fast enough to keep the temperaure from compromising the integrety of the cable - and an open will result. The other risk, is that the insulating jacket is rated for a specific temperature - if this is exceeded internally, the jacket will melt losing the insulating characteristics (thinning, or dripping off of the cable) and the conductor can short against an exposed ground surface causing arcing, welding, or (if close enough to a wood structure) a major fire.

What you are hitting at Gary, but not recommending - is the use of a current limiting resistor - however, I would recommend that one design the bank-tie with a device like this doing the limiting - not the cable! Just like a fuse, this allows us to select a regulation point and locate that part in a known and suitable location.

The math is very simple for a current limiting resistor - one need to know the expected worst-case voltage and current to be limited to and do some simple math:

For example if you are expecting a differecne between a 12Volt Chassis Battery and a 12Volt House battery of 4Volts, and you want to limit your current to 40Amps the math would be:

4Volts/40Amps = 0.1Ohms (100 milliohms)at4Volts x 40Amps = 160Watts

What you can see here is that value is in the order of milliohms - an undersized conductor can easily have several milliohms of resistance per foot, so the risk is real.

Now that's the subject of current limiting resistors - but just like with high power LEDs (current limiting resistors are fine for an LED with a drive current of 0-20mA), a current limiting resistor quickly errodes the efficiency of a system. As you can see in the above math, the resistor will actually convert 160Watts of your precious power into heat while it is protecting the wire! This is like running your headlights. To maximize the transfer of energy from one bank to the other - one must design the system to support the full surge load of the banks (this can easily mean a current number somewhere in the odds of >500Amps).

Back to the example of the LED, lets assume a 3.25Volt 15mA blue LED (because I think they're cool). If we want to run that LED from a 28.8Volt supply, we need a current limitting resistor which will consume the difference between the 28.8volt supply and the 3.25Volt rating of the LED.

28.8 - 3.25 = 25.55Volts

To pick the current limiting resistor, we do the same math as we did for the bank tie:

25.55Volts / 15Milliamps = about 1.7KOhms

With a power rating of at least:

25.55 x 15mA = 0.383Watts

With a resistor, you always pick the next part UP in size - so this would be a 1.7KOhm 1/2Watt resistor. Looking at the efficiency, we see that the LED consumes (3.25Volts x 15mA =) 0.049Watts and the resistor consumes 0.383Watts out of the total power consumed (0.383 + 0.049 = 0.432Watts) the LED only consumes 11.343% of the total power used for the circuit!! That's almost 10% efficiency and 90% waste as heat!

This same lesson can be applied to the battery bank tie circuit with a wire acting as a current limitting device - you'll probably lose more in the wire loss that you gain with the transfer itself, if you have an under-designed circuit. This has a significant impact for those who boondock or full-time. Careful selection of components and special attention to design have a vast impact in the power consumption of the final system.

And please everyone - Always place a fuse with a value picked to protect the wire it is attached to (and place the fuse within 8-inches of the circuits +supply). And place a fuse or fuse block any time you have a transition from one wire size to another - to protect the smaller of the two wires! Only you, can prevent electrical fires in your rig.

Gary, I certainly agree with Tim on the cautions about fuses and wire sizes. My alternator, like most bus alternators is able to produce about 300 amps. My intertie has a 200 amp circuit breaker and is a 00 wire. I want and need the full 200 amps to run my AC, while driving, from the inverter. As it is my wire size is smaller than I'd like. The burning of the contactors contacts is, no doubt due to the energy stored in the inductance of that 00 wire. Remember above I estimated the energy at 1/4 joule. A watt is a joule per second. If the arc lasts 10 microseconds, a very reasonable estimate, during that time the average power in the arc is 25,000 watts. That 'in a nutshell' is why the contacts burn. RegardsJerry 4107 1120

Hi Gary, I'm gald to hear that . A 100Amp fuse is about the limit for a #6 wire with a 12volt system - depending on the length and whether or not the ambient temps are high (like in an engine compartment). That shouldn't be an issue.

Assuming that you take very good care of your batteries and never let them get dicharged below 50-60%, there shouln't be to much load.

Hi Gary, I'm gald to hear that . A 100Amp fuse is about the limit for a #6 wire with a 12volt system - depending on the length and whether or not the ambient temps are high (like in an engine compartment). That shouldn't be an issue.

Assuming that you take very good care of your batteries and never let them get dicharged below 50-60%, there shouln't be to much load.-Tim

That probably makes the difference right there. Gary knows, and will maintain his system well enough to not put those stresses on it.

Thanks Jay, right on. My first set of batteries was four T-105's that I bought from a Burning Man nutcase for $5. He'd bought them new and abused them so badly in their first week that the posts were melted off!! I spin-welded plugs in the holes and repoured the posts, and the batteries performed flawlessly for 4 years! I finally changed over to a new set last season only because they leaked acid a little bit- you can only do so much to save melted batteries- and they were getting a bit out of hand on corrosion issues. I gave all 4 to a friend who is still using them in his RV today. The new T-125's are happy as can be.

Tim and all, YES I use the resistance of the wiring to the solenoid to soften any massive currents that the solenoid would see. The 100A fuse keeps things safe but since it's never blown it's obvious that the wire is doing it's job happily and currents are well within limits. I still maintain that current is the problem in this thread, not inductance spikes. For heavens' sake, how many systems do you think are out there that join battery banks with a solenoid and how many require diodes to keep in working condition? Likely the answers are, in order, many and zero.

There's something going on with David's system that is out of the ordinary, and I'm willing to bet that it has nothing to do with inductance spikes. It's just too difficult to kill this kind of solenoid especially with a system that is in extremely common usage that has, to my knowledge, NEVER needed spike suppression diodes in the contact side of the system... Solenoids of this type routinely operate starter motors, golf cart motors, which represent massive inductive loads, and they live on happily. On the flip side, running too much current or making/breaking contact into too much current through solenoids will kill these things in a jiffy and I've seen that MANY times. I really think that the diode discussion on this thread is informative, but has little to do with Dave's actual problems.

80-100Amps is the limit for conducting power to a load with only .5Volts drop. If you are never seeing more than 40Amps across the cable, the difference in battery charge is probably more in the area of a volt and less. This is great for your system (and speaks well of your battery state/maintenance), but it also means that current limiting is not being done by the cable (since you're nowhere near the capacity for that gauge wire).

Current golf-cart/forklift contactors rarely ever see a switching instance under load - the speed controllers have internal transistors/MOSFETs which do the load switching.

The surge-load topic I brought up way in the beginning of this thread is also something to return to. If the contactor is being opened or closed at the same time as the starter solenoid is being opened/closed - this will cause a HUGE kick (since the load increases dramatically when the starter is turning. If you tie the batteries together - it should be done before the starter is enabled, and should remain tied until after the starter is off for a bit.

I installed the diode as below. I cleaned the solenoid and it had rust in it. Somehow moisture had penetrated the shell. The 0000 cable on the left is from the alternator. Cable on the right is from the house bank.

After cleaning I had no voltage drop to the house bank. I ran the rooftop AC with no trouble. Hopefully it will stay that way.

Wow, you have been searching way down the board pages to find this and bring it back. I found it made no difference and removed the diode about 3 years ago. The solenoid has worked fine without it. I do have to pull it apart about every 2 years and clean the contacts, but overall, I'm pleased with it.